This invention relates to an improved dispersion managed fiber (DMF), methods of making dispersion managed fiber, and a corresponding apparatus for making such fiber. This invention also relates to a telecommunications link including dispersion managed fiber for transmitting signals in a return-to-zero (RZ) signal format and a corresponding method.
Telecommunications networks often include high powered lasers introducing light signals into relatively small effective area optical fibers. The high light power density in these fibers can cause significant nonlinear effects such as four wave mixing (FWM). In order to reduce these signal degrading nonlinear effects, fibers with sufficient dispersion are used in the networks. The dispersion, expressed in picoseconds/nanometer-kilometer (ps/nm-km), tends to broaden a light signal pulse traveling through the fiber thereby decreasing the light power density and reducing nonlinear effects.
While the introduction of dispersion reduces nonlinear effects in the fiber, the accumulation of dispersion over a long length of fiber can cause overbroadening of the signal pulse and may degrade signal transmission. Thus, the accumulated dispersion (or total dispersion product) must remain below an acceptable value which depends upon the transmission parameters, such as modulation format and data bit rate. The dispersion product for a fiber of length L with a dispersion D is the product of L and D, i.e., Lxc2x7D. Thus, the dispersion product of a length of fiber having individual sections of length Li and dispersion Di is the sum of the individual dispersion products xcexa3 Lixc2x7Di.
To overcome problems with accumulated dispersion, dispersion is periodically compensated for by introducing dispersion of opposite sign to the accumulated dispersion along the fiber path. In this manner the accumulated dispersion stays below an acceptable value, while on average the dispersion is non-zero, and reduces nonlinear effects.
Dispersion-managed fiber (DMF) is a continuous fiber comprising alternating sections of positive and negative dispersion. These extra design parameters can be used to great advantage to improve the optical data transmission of both non-return-to-zero (NRZ) and return-to-zero (RZ) data modulation formats. For NRZ, high ( greater than 5 ps/nm-km) local dispersion reduces FWM while a near zero average dispersion maintains the original shape of the light pulses in the data stream. For nonlinear RZ, the pulses react to high local dispersion by broadening and recompressing. This breathing property reduces or eliminates nearly all degrading effects of distributed fiber loss and discrete, periodic amplification. Although these benefits are known, most attention has been placed on managing dispersion discretely at each amplifier or at the transmitter and receiver in the transmission system.
Dispersion slope is another dispersion parameter which must be taken into consideration for multichannel transmission over a range of light wavelengths. Dispersion slope is the change in the dispersion with change in the transmitting wavelength. If the dispersion slope of the transmitting fiber is not zero, the dispersion will be different for different channels with different wavelengths transmitted. Thus, any dispersion compensation scheme which compensates for the entire wavelength range of a multichannel transmission, must take into account the different dispersion at different wavelengths.
Many prior art dispersion-managed systems consist of discrete sections of constant dispersion fiber fusion spliced together into a long-length fiber link. Examples include standard non-dispersion shifted fiber together with dispersion-compensating modules and dispersion-managed cable with fibers of alternating dispersion sign. Methods of making continuous DMF have also been proposed, such as those disclosed in U.S. Pat. No. 5,849,537 to Berkey et al., which is incorporated by reference. These proposed methods include: 1) longitudinally varying the composition of the soot deposition (or chemical composition in the case of sol gel), 2) reducing the core cane preform diameter by, for example, heating and stretching, grinding and polishing, chemical etching, or laser ablation, 3) decomposing individual cores via dicing and polishing or scribing and breaking and reassembling them into a composite core by insertion into a cladding glass tube, and 4) inducing cladding diameter variation during fiber draw.
Processes 2 and 4 use the same core index profile for both the positive and negative dispersion sections and rely on a diameter stretch factor to achieve the dispersion variation. The index profile is the refractive index as a function of the radial distance from the axis of the fiber. In the profiles disclosed in Berkey et al. a diameter change of 10 to 15% is sufficient to achieve the magnitude of dispersion variation required for many applications where the positive dispersion corresponds to the larger core region. Processes 1 and 3 allow for the individual control of positive and negative dispersion refractive index profiles.
Sharp dispersion transitions have been thought necessary for NRZ systems because more gradual dispersion transitions have been thought to allow for FWM to build up in NRZ systems. Gradual dispersion transitions have also been believed to be detrimental to RZ systems.
Of processing techniques 1-4, only one produces sharp transitions between the dispersion sections, process 3. It is also possible with process 4 but this process could be limited to slow draw speeds. As a result, processes 3 and 4 have been the focus of research.
It is unclear, however, if the methods for producing fibers with sharp transitions are appropriate for high speed DMF manufacturing. Process 3 requires physically disassembling individual canes and reassembling them into a composition cane. The waveguiding region requires an additional surface polishing and a cleaning step to avoid formation of defect seeds. It remains unclear if this can be done repeatably in a manufacturing environment. Method (4) may introduce more gradual transitions either during processing steps, such as cane stretching during redraw, or in later steps, such as with laser ablation, due to diffusion or glass reflow. Even changing the cladding diameter on the draw may not provide sharp enough transitions for the draw speeds desired in production.
An advantage can be achieved if DMF with smoothly varying dispersion maps can be used for nonlinear RZ transmission. The present inventors have discovered, that indeed, for nonlinear RZ transmission, DMF with smoothly varying dispersion maps compare quite favorably with those having sharply varying dispersion maps, such as square wave maps. The present inventors have found that for DMF with smoothly varying dispersion maps, the more gradual dispersion change through the zero dispersion region does not adversely affect transmission. Appropriate smoothly varying dispersion maps include sinusoidal, saw-tooth, trapezoidal, or more arbitrary dispersion maps. An advantage of dispersion maps that do not vary sharply is that the DMF may be processed by a wide variety of techniques, including those that provide a high fiber production rate. Processing techniques not previously thought suitable for producing DMF for NRZ applications have now been found to be acceptable.
According to a second and third embodiment of the present invention, methods of forming DMF by localized heating and cooling, respectively, are provided. The localized heating or cooling methods are suited to forming DMF with either smoothly varying dispersion or sharply varying dispersion. An advantage that can be achieved with the second and third embodiments using localized heating or cooling is that the thermal effect is nearly instantaneous and provides discrete diameter changes without overshoot or ringing. A further advantage that can be achieved with localized heating or cooling is that it is relatively simple to retrofit existing manufacturing draw equipment without affecting other processing or control loops.